Final answer:
The tissue flap over an earthworm's mouth is known as the prostomium, which does not function in digestion but assists in sensing the environment and burrowing.
Step-by-step explanation:
The flap of tissue that covers the earthworm's mouth is called the prostomium. This structure serves as a sort of lip that helps the earthworm sense its environment and also aids in digging through the soil.
Earthworms do not possess a mandible or a proboscis, and they certainly don't have a gizzard at the anterior end where their mouth is located. Instead, the gizzard is part of the earthworm's digestive system further down the tract where it plays a role in grinding up the soil and organic material the worm ingests.